Hi GF,
Great pictures, especially that sharp, high contrast image of the garter snake's head. Love that eye.
I was previously aware of the parietal (=pineal) eye in unusual reptiles, like the tuatara. In the tuatara, the eye appears to have not only have neuroendocrine functions a semi-retina like structure (see
here and
here). In the tuatara, this third eye also appears to have a
neuroendocrine function, much like the pineal gland and it may have some
photoreceptor function. In lizards that have a parietal eye (and some other vertebrates, such as some amphibians, and some fishes), this structure primarily seems linked to the regulation of melatonin (and perhaps other hormones) by the pineal gland to which it is attached but not in vision.
Several sources state that as a group snakes lack a parietal (=pineal) eye (see
here and
here and
here). They have pineal glands, but these are part of the brain, not penetrating through a scale. Also, I looked at other
pictures (see
here too) of various garter snakes, I could not find anything similar to the dot in the frontal scale which you highlighted with the red line in your image. Might you have a well-positioned dew drop?
Steve